grib_io.c: restore signedness for arguments for seek

stdio.seek, which calls fseeko, uses "off_t" for argument, and
when seeking toward head of the file, passing negative off_t value
must be passed.

Currently when callling stdio.seek, "size_t" variable is passed
to the argument. On 64 bit system, the problem is hidden because
both size_t (unsigned) and off_t (signed) are 64 bit.
But on 32 bit system, as size_t is unsigned 32 bit and off_t is
signed 64 bit, when passing size_t "message_size - already_read"
(which is intended to be negative to seek toward head) is converted
to large unsigned 32 bit value, which is not intended.

The fix is to correctly cast to off_t.

The attached patch fixes the following tests failure on 2.4.1:

# 100 - eccodes_t_bufr_ecc-875
# 118 - eccodes_t_gts_get
# 119 - eccodes_t_gts_ls
# 120 - eccodes_t_gts_count
# 121 - eccodes_t_gts_compare
# 122 - eccodes_t_metar_ls
# 123 - eccodes_t_metar_get
# 124 - eccodes_t_metar_dump
# 125 - eccodes_t_metar_compare
# 162 - eccodes_t_grib_copy